In an aircraft gas turbine engine fuel supply system it is common to use a positive displacement pump such as a gear pump or a piston pump to provide fuel at high pressure for supply, through a fuel metering unit (FMU), to the burners of the gas turbine engine. Positive displacement pumps all have an individual internal leakage characteristic in that some proportion of the high pressure output flow of the pump leaks back to the low pressure inlet side of the pump by way of internal leakage flow paths in the pump. Such leakage becomes progressively worse with time as a result of wear of the internal components of the pump. Clearly pumps will wear at different rates dependent upon their usage and the ambient conditions in which they operate. Eventually a point is reached in the service life of every pump at which the leakage is sufficient to prevent the pump being able to deliver either the flow required to start the associated engine at low engine/pump speeds, or provide the flow required for aircraft take-off at maximum engine/pump speed.
Since the rate at which a pump wears is dependant upon its operational parameters it is extremely desirable to be able to monitor the health of the pump in use, that is to say to monitor its leakage characteristic, during service, in order to allow pumps to be taken out of service at the optimum time in relation to the life of the pump rather than at fixed maintenance intervals. Thus a pump could be replaced when its monitored health indicates that a failure may be imminent rather than replacing the pump at a fixed and pre-determined time. Naturally pumps which are exhibiting less wear can remain in service longer.
It will be recognised that, ignoring leakage in the pump, the output flow of a positive displacement pump is proportional to its rotation speed. Generally, positive displacement pumps in aircraft engine fuel supply systems are driven from an engine accessory gear box so that the pump speed is directly proportional to the speed of the associated engine.
In the specification the term “flow number” is used. It is believed that the term is well understood in the art, but for the avoidance of doubt a flow number of an orifice, or flow path, is a measure of the flow restriction which that orifice or path presents at a given hydraulic pressure drop across the orifice or flow path. The flow number of an orifice or flow path is calculated by dividing the flow through the orifice or other flow path by the square root of the pressure difference across the orifice or flow path.